One of the (usually unstated) assumptions of the drug warriors here on FR is that America is just chock full of would be drug users who are only deterred by the law.
That if drugs were decriminalized, everyone up and down your block - and mine - would rush out to stone themselves into a frenzy or a stupor (depending upon the chemical of choice).
I don't buy that argument, and I submit that if this is true, we have a much bigger problem in this country than whether or not some drugs are illegal.
To your point, if we legalized meth, my guess is that over time the percent of Americans who are meth users might go up, or go down, by a statistically insignificant amount.
The problem is not drug USE, or Alcohol USE, it is ABUSE.
Same is true of GUNS.